1. Field of the Invention
This invention relates to a phase detector, particularly though not exclusively to a phase detector for use in a phase locked loop which may be used in a frequency synthesiser system.
2. Description of Related Art
The phase locked loop frequency synthesiser is a system, as shown in FIG. 1, whereby a variable frequency oscillator 2 can be phase locked by a phase comparator 4 to a fixed and stable reference oscillator 6. By connecting a variable ratio divider 8 between the variable oscillator 2 and the phase detector 4, the phase locked frequency of the variable oscillator 2 can be modified in response to a digital word from a microprocessor 10 or other data source. A reference divider 12 is coupled to the reference oscillator 6. The output of phase detector 4 is coupled via a loop amplifier 14 to control variable frequency oscillator 2.
When a phase locked loop is first switched on, the inputs to the phase detector will not be phase locked or even at similar frequencies, and, therefore, to allow the loop to rapidly acquire lock, a phase detector which is phase and frequency sensitive is desirable. A well known detector as shown in FIG. 2 with the necessary characteristics comprise two flip-flops 20 having their outputs coupled via an AND gate 22 to reset inputs at flip-flops 20. This type of detector produces high or low outputs depending on the phase/frequency relationships of the input pulses, but always attempt to align the input edges for zero phase error once locked. When this condition is achieved there is a non linearity in the transfer characteristic, shown in FIG. 3, which results in the oscillator being uncontrolled in the region of zero phase error.
This disadvantage is often overcome by the addition of an analogue phase detector which is linear in the zero phase error region, but is not capable of pulling the loop into lock from far off as it is not sensitive to frequency. See for example the Plessey NJ8821 series of Frequency Synthesisers, described in pp236-245, 306-312 "Frequency Dividers and Synthesisers", Plessey Semiconductors, 1988. The digital detector is used to achieve rough phase lock, and then an automatic switch-over made to the analogue detector to maintain fine control. A requirement of an analogue phase detector for use in conjunction with a digital detector is that both comparators bring the outputs from the reference and variable frequency oscillators to the same phase relationship to avoid glitches when the switch-over is made. Thus as shown in FIG. 1, phase detector 4 comprises a digital detector 14 and an analog detector 16, the outputs from the detectors being applied to an amplifier 18, provision being made (not shown) for ensuring detectors 14 and 16 do not operate simultaneously.